In this connection the invention shall include disc brakes which either comprise a sliding caliper or a fixed caliper, which overlap one or more brake discs. Mainly but not exclusively the invention relates to disc brakes with lining sections of the spot-type.
Disc brakes, in particular for heavy load trucks, are known with different configurations, both with respect to the type of the actuation mechanism, with respect to the way of the transmission of the braking or clamping force onto one or several brake discs and with respect to the type of the adjustment for compensating the wear of the brake pad and/or the brake disc.
Generally, there exists a wide range of actuation mechanism and brake adjuster designs applied and embodied in disc brakes.
For example, from EP 0 271 864 B1 an actuation device is known which comprises an axial actuation member around which several components are arranged such as a force amplification mechanism in the form of a roller-ramp mechanism. Commonly used actuation mechanisms do embody brake levers which are actuated by hydraulic, pneumatic or electro-mechanic actuators, which lever is transmitting the introductory force usually by means of one or more roller elements to a thrust piece, which is mainly axially guided in the housing of the brake caliper towards the brake disc. Such lever-actuated brake mechanisms are e.g. known from EP 0 553 105 B1 or EP 0 698 749 B1.
An alternative design of a brake actuation mechanism is shown in WO 2001/075324 A1, in which the thrust element for transferring the clamping force onto the brake disc is arranged around a central rod. A similar design is e.g. known from WO 2004/059187 A1 of the applicant. The brake actuation mechanism for a disc brake as disclosed therein comprises the shape of one single tappet and thrust element, respectively, which acts onto the brake disc, in which an adjustment device is already integrated within the tappet. The tappet thereby is mounted in the housing of the brake caliper in parallel to the rotary axis of the brake disc by means of a rod. According to a preferred embodiment the tappet directly cooperates with a return mechanism for the thrust element, which is integrated in the tappet and which cooperates with the rod.
Independent from the design of the actuation mechanism to be employed in this connection, a compensation of the running clearance between the brake disc and the brake lining at the brake pad, which results from the wear at the brake pad lining and/or at the brake disc, has to be always provided. The brake actuation mechanism according to this example of a tappet as mentioned herein at least comprises a thrust element which transmits the clamping force onto the brake disc by means of the brake pads. For that purpose the thrust element comprises at least one rotatable element which interacts with a further element, in which the latter element is guided in the brake caliper in a non-rotatable manner, however axially displaceable, so that both elements can be axially displaced in relation to each other.
By means of an adjustment mechanism having at least one clutch element, which comprises some degree of lost motion, in which the level of free motion is selected to correspond to the desired target clearance, i.e. the maximum allowable running clearance, the friction lining of the brake pad, e.g. the brake pad itself, will be progressively advanced towards the brake disc as the friction lining slowly decreases due to wear. Namely, if the friction linings do wear to such an extent that the running clearance is greater than the target clearance, the existing running clearance between the brake pad and the brake disc is taken up and the further additional movement of the brake pad towards the brake disc, which results from and thus corresponds to the actually existing excessive lining wear, causes the clutch to rotate, which rotation will advance the brake pad towards the brake disc. After contact of the brake pad with the brake disc, the increased load in the entire brake mechanism will cause the clutch to slip, thereby preventing further, now not required adjustment movements or any overloading of the adjustment mechanism.
As an example such principle is realized by WO 2011/113554 A2 of the applicant in which two sleeve-like spindles are in a threaded engagement, in which an external sleeve having an internal thread receives an internal sleeve having an external thread in a rotational manner. The external sleeve or outer spindle is supported in the brake caliper or in its carrier in a non-rotatable way, so that rotation of the internal sleeve or inner spindle results in a linear displacement of the outer spindle in relation thereto, so that the outer spindle can be moved towards the brake disc in order to compensate the wear-induced clearance at the brake linings. For example, the non-rotatable linear guidance of the outer spindle in axial direction can be realized in that the outer spindle is connected either directly or by means of further intermediate elements with the brake pad or the brake lining retainer, which in turn themselves are guided in the brake caliper or in the caliper carrier in a linear manner.
The single tappet-type actuation mechanism as known from WO 2011/113554 A2 shows an enhanced design, in which the elements of the return mechanism and the adjustment mechanism are incorporated into the tappet design and surrounded by the unit consisting of the outer and inner sleeves, which both form part of the adjustment mechanism and thus the brake actuation mechanism. The adjustment mechanism incorporated therein is driven by the rotatable lever, which introduces the clamping force into the brake actuation mechanism. Furthermore, the adjustment mechanism comprises a torque limiting clutch mechanism slipping or overrunning when the rotational resistance between the threaded parts rises due to the axial force, which is built up when the brake pad contacts the brake disc, thereby creating some kind of counter-torque. The torque-limiting clutch mechanism may be configured as a roller-ramp mechanism. In addition, an one-way clutch element may be provided, over which the torque from the lever will be transmitted in its driving direction.
For automatic brake adjusters it is furthermore known to combine the torque limiting clutch function with an one-way clutch function in one component, as this, for example, is known from U.S. Pat. No. 4,527,683, which discloses a wrap spring which comprises a double function, namely meaning that it is configured to provide a torque limiting clutch function on the one hand and an one-way clutch function on the other, which in fact is achieved in that at one end of the wrap spring the inner contour or section of it applies the one function, either the torque limiting function or one-way function, while at the other end of it its outer contour or section applies the other function, accordingly.
The advantage to combine these two functions by one element, however, is accompanied by the drawback that a rather large friction torque in the free-running direction of the section aiming at the one-way clutch function has to be provided which is due to the fact that the dimensions of the wire used for winding the wrap spring have to be selected to be rather large, so that a high enough and well defined driving torque at brake actuation can be transmitted in any case.
Furthermore, in principle, it is required that the friction torque in the free running direction of rotation for the one-way clutch function should be less than the frictional resistance resulting from all other moveable members of the adjustment mechanism in order to avoid any backward rotation of the adjustment mechanism at brake release.
In fact, at brake release, i.e. when the brake pad loses contact with the brake disc and no more braking force is transferred from the actuation mechanism to the brake disc, automatic brake adjusters, which do employ the afore-mentioned double function wrap spring, under certain circumstances could then tend to rotate in the other direction for some small degree, which could lead to the effect that the adjustment mechanism will drive some components of the brake actuation mechanism, in particular the outer sleeve, backwards again to some, even though small extent.
Accordingly, there exists the drawback that the slack, which corresponds to a decrease of the wear-induced distance between the brake disc and the brake pad/lining, as previously set by the adjustment mechanism during brake actuation will be again increased, even though to a minor extent as compared to the clearance reduction, by the return motion inherently resulting from the torque limiting clutch mechanism. The problem of some undesired backlash motion, anyway, more or less exists for all kind of automatic brake adjusters in the prior art.
It may be possible to lower such tendency of backlash motion applied by the wrap spring during brake release by tilting the wire of a rectangular section or by grinding down the wire sections at one end. However, considering the forces and torques being present in disc brakes such measures usually have proven to be not enough, so that there may be the further need to introduce an extra friction resistance somewhere between components of the adjustment mechanism or of the brake actuation mechanism which are moveable relative to each other. In turn, however, such increase of the inherent frictional resistance in the mechanism would again require a higher driving torque from the entire torque limiting clutch mechanism, which may call for larger wire dimensions, which, however, will then counteract the need to have a free-running torque in the one-way clutch section of the wrap spring which shall be as low as possible.